1. Field of the Invention
This invention relates to a contact combustion type carbon monoxide sensor which can detect carbon monoxide (hereinafter referred to as "CO") with a high sensitivity at a bridge voltage of 6 to 12 V but hardly has any sensitivity to gases other than CO.
2. Description of the Related Art
CO gas has no color, no taste and no odor and is somewhat lighter in weight than air. When a man breathes for 2 to 3 hours at a CO concentration of 200 ppm, he will surfer from a headache. CO gas has such a high toxicity that when a man breathes for 2 hours at a CO concentration of 1,600 ppm, he will be dead and at a concentration of 3,200 ppm, he will be dead within 10 to 15 minutes.
Standards or required characteristics for CO gas sensors in various countries are generally such that the sensors beep an alarm at a CO concentration of 200 ppm, do not beep any alarm at a H.sub.2 concentration of 500 ppm and a C.sub.2 H.sub.5 OH concentration of 1,000 ppm, and do not have any sensitivity to the other gases. Furthermore, CO gas sensors must not involve an erroneous operation due to the change of a moisture or a temperature or due to the change of a power source voltage, must have high reproducibility and must be free from the change with the passage of time.